The present invention relates generally to melter kettles that are designed and used to melt thermoplastic materials that are applied to pavements such as roadways, airport runways, parking lots, bicycle paths and other surfaces requiring pavement markings. More particularly the present invention is directed to systems and methods to improve the melting efficiency of melter kettles.
Thermoplastic materials are the product of choice for many types of pavement marking operations. However, unlike most other types of marking materials thermoplastic materials must be melted to very high temperatures that can reach up to about 420° F. in order to be fluid enough to be applied.
Early types of thermoplastic material application equipment applied the thermoplastic material at slow rates. Therefore, the long melting times it took to melt thermoplastic materials in melter kettles were not a problem. Melter kettles could keep up with the low output of application equipment.
Eventually improvements in the designs of melter kettles achieved reductions of melting times. However, over time application equipment was improved to the point at which thermoplastic material could be applied at much faster rates than the improved kettles could keep up with melting the thermoplastic material. The present invention increases the efficiency of melting thermoplastic in melter kettles that can be mounted on either thermoplastic trucks, nurse trucks, trailers or the like.
For some time heat domes, also called heat risers or heat tubes, have been installed in melter kettles. The dome structure is formed by a tube of variable diameter that is attached to a hole in the base of a kettle where the OD of the dome base matches the ID of the hole in the base of the kettle. The top of the dome is closed by a metal disc. The dome reduces the heating surface area of the base. However, the dome provides additional circumference surface area that compensates for the loss of the heating area in a kettle with no dome and compensates for the lost surface area of the base within a few inches of dome height. From this point the dome adds melting (heat transfer) surface area to the kettle with a dome as compared to a kettle without a dome thereby increasing the overall heating surface area in the kettle that acts on thermoplastic material in the kettle. This reduces the ratio of thermoplastic material to melting (heat transfer) surface area of the kettle which improves heating efficiency. Additionally, heating thermoplastic material in a melter kettle from the middle of the kettle in an outwardly direction is more efficient than heat transfer from the outside of the kettle in an inward direction. Heat domes have reduced melting times in kettles. However, heated air in the dome cools as heat transfers through the dome wall and into the thermoplastic kettle. Melting times are reduced with the use of domes but still needed to be improved.
A recent improvement in melter kettle efficiency has been developed by the present inventor and is disclosed in U.S. provisional application Ser. No. 62/291,316, entitled “HEAT DOME TEMPERATURE REGULATING SYSTEM,” filed Feb. 4, 2016. In this copending application a heat dome chimney stack tube is attached to the top center of a heat dome about which an agitator drive shaft tube rotates. Hot combustion gases travel from the heat dome up the center of the heat dome chimney tube stack and vent into a top tube drive shaft heat chamber that has driveshaft tube relief vents through which combustion gases vent into the atmosphere. The venting of the combustion gases can be regulated by providing a rotational vent relief collar about the top tube drive shaft heat chamber. This system exhausts combustion gases from the dome that has been heat depleted thereby allowing a continual flow of hot combustion gases to maximize/optimize efficient temperature in the dome such that the maximum amount of heat is transferred through the dome and chimney stack surface areas into the thermoplastic material in the kettle. In this system the heat dome chimney stack tube and rotational drive shaft become heating surfaces through the centerline of the melter kettle. This system improves the rate of thermoplastic melting.
Another recent improvement in melter kettle efficiency developed by the present inventor is disclosed in U.S. provisional application Ser. No. 62/322,640, entitled THERMOPLASTIC MELTING KETTLE MATERIAL CIRCULATION SYSTEM, filed Apr. 14, 2016. In this improvement a single vertical material transfer tube is affixed to the side of the thermoplastic melter kettle either directly to the kettle side wall or to the outer insulation skin. The tube is attached to ports at the bottom and top of the melter kettle and an auger rotated by a direct drive motor within the vertical material transfer tube moves molten material from the bottom of the kettle to the top. When the vertical material transfer tube is connected directly to the kettle outer wall the bottom interface is within the heat chamber's outer wall.
When the vertical material transfer tube is affixed to the outer insulation skin there is an extended heat chamber surrounding the vertical material transfer tube. A port larger in diameter than the lower material transfer tube allows heat from the combustion chamber to contact the vertical material transfer tube.
Another recent improvement in melter kettle efficiency developed by the present inventor is disclosed in U.S. provisional application Ser. No. 62/291,309, entitled THERMOPLASTIC KETTLE AUXILIARY HEAT EXCHANGER SYSTEM, filed Feb. 4, 2016. This invention combines an odd number of interconnected vertical tubes within an oil bath through which heated heat transfer oil flows. The function of the system is to increase the temperature of molten thermoplastic moving through the circuit of interconnected heat transfer tubes by action of an independent high BTU output furnace that heats circulated heat transfer oil that circulates around the interconnected tubes. Molten thermoplastic enters the base of the first tube through a kettle bottom material flow port and the tube bottom material flow port both of which are isolated from the oil bath. The molten thermoplastic reenters the kettle at the top center through the top flow tube that connects to the top of the discharge tube that is above the level of the kettle top and is isolated from the oil bath. Each tube contains an auger. The augers are interconnected by a gear train. A single hydraulic motor attached to any auger drives each gear and auger in a counter rotational direction. This circulates the molten thermoplastic material from the bottom of the kettle where it is hottest through the kettle bottom material flow port into the bottom of the first tube then up and down the plurality of tubes. The material flows up the last tube and through a tube top port which is isolated from the oil bath and through the top material flow tube located at a level above the top of the kettle. The molten thermoplastic is deposited near the top center of the kettle where it heats and displaces downward the thermoplastic at the surface of the kettle. The heat transfer oil enters the oil bath tub adjacent the thermoplastic discharge port where both the oil and thermoplastic are at their hottest temperature and is directed through and leaves the system adjacent the thermoplastic inlet port where it is heat depleted. When the system is disengaged and circulation ceases the hydraulic motors are run in a reverse direction to purge as much thermoplastic from all tubes except for the inlet tube. This will leave solid material in only the first tube so that when the system is restarted it will take less heat and hydraulic energy to engage the system and begin moving molten material.
There is a limit to the various available energy outputs of mobile equipment systems that can be incorporated in thermoplastic equipment such as heat, electrical, engine, hydraulic air and other systems. Some serious draw backs to thermoplastic oil bath auxiliary heat exchanger systems are that they require a separate high BTU boiler system, separate hot oil circuits as well as oil expansion chambers designed for use with exotic heat transfer oils some of which require inert gas blanket interfacing. The high BTU output boilers required need more space than is available on most thermoplastic application trucks. Where they can be used they require special designs and fabrication. Motors to run the hydraulics and oil circulation systems also are subject to space limitations. Weight is also a serious consideration. For each pound that the system weighs the carrying capacity of the thermoplastic application truck is reduced by a similar amount. Costs are high for all of the system components.